Spray drying method and apparatus for producing granular particles from stock liquids of solids

ABSTRACT

A spray drying method comprising forming a layer of solid particles at the bottom of a drying chamber defined by a vertical cylinder, blowing gas into said layer from the underside to agitate the whole of said layer and thereby to form a fluidized bed of solid particles with part of said particles being blown up from the upper surface of said fluidized bed and suspended within said drying chamber, blowing gas into said drying chamber in a direction to form a swirling downward gas flow along the side wall and a swirling upward flow along the center of said drying chamber, whereby said suspending particles are caused to make a circulatory motion while being entrained in said swirling gas flows in such a manner that the fine particles spirally move upward in the center of the drying chamber and at least the major part thereof approach the side wall of the drying chamber under the effect of centrifugal force before they reach the top of said drying chamber and finally spirally move downward along the side wall of said chamber, said particles being partially returned to said fluidized bed and partially being carried upward again in the center of the drying chamber by being entrained in said swirling upward gas flow, spraying stock solution into said drying chamber, the droplets of said sprayed stock solution colliding against and adhered to said circulating particles to form larger sized particles, said larger sized particles being dried by the surrounding gas and partially circulated within said drying chamber and partially falling into said fluidized bed, and discharging said larger sized particles from said fluidized bed while maintaining the depth of said fluidized bed substantially constant.

Asizawa et al.

[ 1 May 29,1973

54 SPRAY DRYING METHOD AND Primary Examiner-Norman Yudkoff APPARATUS FORPRODUCING Assistant Examiner-J. Sofer GRANULAR- PARTICLES FROM STOCKAttorney-Heir, Gipple & Jacobson i LIQUIDS OF SOLIDS 57 ABST C [75Inventors: Naohito Asizawa, Sugihami-ku, I RA T Motomi Kono Edogawzbku,A spray drying method comprising forming a layer of Tokyo, boy}, f Japansolid particles at the bottom of a drying chamber defined by a verticalcylinder, blowing gas into said [73] Asslgnee: Tekk Kabushlk' Ka'shaslayer from the underside to agitate the whole of said Tokyo, Japan layerand thereby to form a fluidized bed of solid par- [22] Filed; Man 4 1971ticles with part of said particles being blown up from a the uppersurface of said fluidized bed and suspended PP 120,922 within saiddrying chamber, blowing gas into said drying chamber in a direction toform a swirling [30] Foreign Application Priority downward gas flowalong the side wall and a swirling upward flow along the center of saiddrying chamber, dune 1,1970 Japan ..i ..45/47026 whereby aid suspendingparticles are caused fo'make a circulatory motion while being entrainedin said Y U- Clam! 159/48 3 swirling gas flows in such a manner that thefine parti- [51] I Int. Cl. ..,...B0ld 1/16, F26b 3/12 cles spirallymove upward in the center of the drying [58] 'Field Of Search...- .l.'......l59/4, 4 CC, 4 E, chamber and at least the major part thereofapproach I 3 1316- 57 E the side wallof the drying chamber under theeffect of v centrifugal force before they reach the top of said dry-[56] References Cited ing chamber and finally spirally move downwardalong a the 'side wall of said chamber, said particles being par- UNlTEDSTATES PATENTS tially returned to said fluidized bed and partially being1,829,477 10/1931 Douthitt ..lS9/4 Carried upward again in the center ofthe drying 2,357,648 9/1944 Hall ..l59/48 RX chamber by being entrainedin saidswirling ugward 2,561,394 7 1951 Marshall ..159/48 R x 8 SprayingStock 801mm" sald 'Y 3,036,338 5/1962 Naek.....l. ..ll7/l00X Chamber,the droplets Of Said p y Stock Solution 3,415,665 12/ I968 Hussmann..l59/4 CC X colliding against and adhered to said circulating parti-2,054,44l 9/1936 Peebles ..l59/4E cles to form larger sized particles,said larger sized 2, 8 1 /193 Peeb es --l 7/ 6 particles being dried bythe surrounding gas and par- 2i316i207 1 4/ 1943 Wint" 34/57 5 X tiallycirculated within said drying chamber and par- 2635534 4/1953 "159/48 Rtially falling into said fluidized bed, and discharging "159/48 R X saidlarger sized particles from said fluidized bed while 31309362 CWelandm""162/30 maintaining the depth of said fluidized bed substan- 3,376,l244/1968 Hollingsworth' ..7l/39 i tlally constant. i

8 Claims, 1 Drawing Figure DAMPER 35 GUIDE BLADE 2", STOCK DAMPER 5PATENIEWY29 w 3'. 735.792

IQQ I8 HOT GAS INJECTlON NOZZLES l6 v |3 4 |5 SECONDARY AIR HTRSEPARATOR-IO i-I j; DAMPER l l 35 GUIDE BLADE CYLINDRIQAL TUBE 9 f 8 24STOCK 2e 25 E5 X A i [POWDER OR WORKING GAS SEED PARTICLES VALVE 29 a:PRIMARY AIR 2? fk 4 28 3 3o DAMPER I 5 6 NR Naohilo Asizava PROD I MohKano INVENTORS ll SPRAY DRYING METHOD AND APPARATUS FOR PRODUCINGGRANULAR PARTICLES FROM 'STOCK LIQUIDS OF SOLIDS into the cylinder toform a swirling downward gas flow therein and a gas outlet port at thetop end thereof, by injecting air into the cylinder from the bottomthereof through a perforated plate to form a fluidized particle bed of apredetermined height at the bottom of said cylinder, while injecting agas, such as hot air, into said cylinder from the upper portion. thereofthrough said gas inlet port so as to form a swirling downward gas flowalong the peripheral wall of said cylinder and a swirling upwardgas'flow in the center of said cylinder above said fluidized particlebed thereby to circula'torily suspend particles within said cylinder,spraying stock liquid of a solid of the same material as said particles(hereinafter referred to as a stock solution) in said cylinder anddrying the same therein.

-In forming powdery particles from stock solutions, a so-calledspraydrying method has widely been used, in which stock solution is sprayedin a large-sized hot air chamber through nozzle or centrifugal spraydisc means and the resultant droplets of the solution are dried by hotair. Where the spray drying method cannot be employed due to theproperties of stock solutions to be processed, there has been employed amethod in which crystals are formed in a stock solution by thecrystallization method and said crystals are separated from the stocksolution and dried to obtain crystalline particles, or a method in whicha liquid is concentrated by heating, followed by cooling andsolidification to obtain a' solidand said solid is pulverized intopowdery particles. The spray dryig method is advantageous in that, sincea liquid is sprayed directly into hot air, the

droplets of the liquid can be dried in a short period of time-(aboutseconds). However, liquids which can be sufficiently dried in such ashort period of time to enable stable powdery particles, are limited.

What is most important in obtaining a powder by the spray drying methodis to, prevent insufficiently dried,

- highly adhesive particles of liquid fromattaching to the peripheralwall surface of a drying chamber. When the undried particles are broughtinto direct contact with the wall surface of the drying chamber, theyare usually v readily bonded to the wall surface, whereby the yield ofthe product particles is markedly lowered. Furthermore, the accumulationof the particles on the wall surface occasionally necessitatesinterruption of the operation; making a continuous operation impossible.It is, therefore, essential that the droplets are completely driedbefore they reach the wall surface, and to this end, the drying chamberis naturally required to be extremely large in size. This is one of thedisadvantage of the spray drying method. For the same reason as setforth above, it is economically impossible to reduce the water contentof the particles or to completely dry the same only by spray drying, andit is a common knowledge that the drying of particles is effected in theconstant-rate drying period" which is a terminology specifically used inthe art. Therefore, secondary drying becomes necessary for suchmaterials which are required to be dried to a low water content, and arotary drier, a fluidized bed drier, etc. have been used for thispurpose.

Furthermore, since the drying is required to be accomplished in a shortperiod of time, a large quantity of air and heat are required and theheatconsumption is quite large, adding to the construction cost as wellas the operational cost. With reference to the particle size, it isextremely difficult to obtain large sized particles, by reason of thenecessity of accomplishing the at largest, it being usual that the sizeof the product particles is from 100 to 200 mesh. Additionally, sincethe particles obtained by the spray drying method are hollow sphericalparticles, they are not only vulnerable but also small in apparentspecific gravity and hence bulky, which is disadvantageous in packagingand transportation.

A further disadvantage of the prior art spray drying method is that inmolding a ceramic powder produced by the method, a large sized pressingdie is required.

It is also to be noted that there aremany materials, such as a sugarstock solution, which cannot in any manner be transformed into solidpowders by the piror art spray drying method.

As stated above, the prior art spray drying method has a number ofdisadvantages. The present inventor conducted a lengthy study andexperiment with a view to obviating the above-described disadvantages ofthe prior art spray drying method and finally has achieved the presentinvention.

The object of the invention, therefore, is to reduce the size, and hencethe construction and operation costs of the conventional spray dryingapparatus; to make a continuous stable operation possible by readilydrying a stock solution to a degree of dryness which could not beattained by the conventional method, while preventing scattering of thefine powdery particles to the outside of the apparatus and attachmentand growth of the undried droplets of the solution to the wall of adrying cylinder, and to obtain porous granular particles of uniform sizeand large apparent specific gravity which could not be attained by theconventional method; and to make it possible to process stock solutions,such as a sugar stock solution, which could hardly be transformed intopowder heretofore, and thereby to obtain quality granular products.

The object of the invention 'set forth above can be attained by blowinga gas, e.g. hot air, into a cylinder from the bottom thereof to form afluidized bed of particles in a predetermined height, blowing a gas,e.g. hot air, into the cylinder through a gas inlet port provided at theupper portion of said cylinder so as to form .a swirling downward gasflow along the wall of said cylinder which induces a swirling upward gasflow in the center of the cylinder, causing the particles suspended inthe cylinder to make a circulatory motion in which said particles aremoved upward in the central portion of the cylinder while beingentrained by said swirling upward gas flow, moved radially in the upperportion of the cylinder under the centrifugal force imparted thereto bythe swirling gas flow and separated from said upward gas flow andfinally moved downward while swirling in the cylinder by being entrainedin said swirling downward gas flow, exhausting the used air to theoutside of the cylinder through an exhaust port provided at the top endof said cylinder, spraying a stock solution in the cylinder in which theparticles are circulated in the manner described, whereby the finedroplets of the solution impinge against and encrust said circulatingfine particles forming particles of increased mass which partially makea circulatory motion while being dried by the surrounding hot air andpartially fall into said fluidized particle bed at the bottom of thecylinder, while on the other hand the undried droplets of the solutionfalling into the fluidized particle bed are mixed and bonded to thefluidized particles to increase the size of said particles and theundried droplets of the solution attaching the wall of the cylinder aredetached therefrom by being scraped by the downwardly swirling particlesalong the cylinder wall in frictional engage ment therewith and arebonded to said particles, the resultant particles being spirally carrieddownward and part thereof being circulated in the cylinder to be dried,whereby larger-sized particles are continuously produced in the cylinderand dried by the gas blown into the cylinder from the underside and theswirling downward gas flow; and discharging the particles thus formedfrom said fluidized particle bed while maintaining the height of saidfluidized particle bed within a predetermined range.

According to the present invention, it is possible to process a stocksolution in a spray drying apparatus which is substantially smaller andinexpensive than the conventional one, and with less heat; to obtain agranular product which is large in particle size and bulk specificgravity and is excellent in flowability, solubility and dispersibility;and to eliminate a classifier, a screening device, a secondary drier andconveyors associated with said devices, which have been necessaryheretofore. Further, the granular products obtained according to theinstant invention are free of contaminates. Therefore, the presentinvention is of great contribution to the production of ceramic powdersfor electronic equipment which are required to be highly precise incomposition, as well as to the production of highly pure materials forchemicals, medicines and powdery foods. In addition, the invention hassuch a remarkable advantage that granular particles can be obtained evenfrom such stock solutions which could not be transformed into powder bythe conventional spray drying method. Namely, one of the characteristicfeatures of the invention resides in the fact that granular particlesare obtained from stock solutions by the combined steps of forming afluidized particle bed at the bottom of a cylinder by charging a powderinto the cylinder and blowing hot gas into said cylinder from the bottomthereof, forming a circulating flow of suspended particles in thecylinder by blowing hot gas into said cylinder in a manner to form aswirling gas flow and continuously spraying a stock solution in saidswirling flow within the cylinder, whereby the particles in the cylinderare bonded with the droplets of the stock solution and increased insize, and the resultant largersized particles fall into said fluidizedparticle bed, and by discharging the larger-sized particles continuouslyfrom said fluidized particle bed while maintaining the height of thefluidized bed constant. Another characteristic feature of the inventionresides in the fact that a gas is blow into the cylinder from a productdischarge port provided at the bottom of the fluidized particle bed,whereby the particles grown to a predetermined size are selectivelyobtained. It should also be noted that according to the invention, theattachment of the stock solution to the wall of the cylinder can beprevented and the thermal efficiency can be enhanced by heating thecylinder externally.

The method and apparatus of the instant invention will be described byway of example hereunder, with reference to the accompanying drawing:

The drawing is a vertical cross-sectional view of an embodiment of theapparatus according to the present invention.

In the drawing, reference numeral 1 designates a vertical dryingcylinder (hereinafter referred to simply as a cylinder) having acircular horizontal cross-sectional shape, 2 a perforated plate disposedat the bottom of said cylinder 1, 3 an air chamber formed beneath saidperforated plate 2, 4 a regulating damper provided in an air duct 7 toregulate the flow rate of air supplied into said air chamber 3, 5 aheater, 6 a blower, 8 a frusto-conical cover connected with the upperend of said cylinder 1 and having an opening centrally thereof, 9 acylindrical tube disposed in the opening of said frusto-conical cover 8,and 10 a particle separating cylinder (hereinafter referred to simply asa separation cylinder) extending upwardly from the opening of saidfrusto-conical cover 8. The tube 9 and the separating cylinder 10 arearranged in concentrical relation to the cylinder 1. Further, the tube 9is secured to the separating cylinder 10 by suitable means not shown.Between the tube 9 and the separating cylinder 10 is formed an annularpassage 11 through which said separating cylinder 10 is communicatedwith the cylinder 1. Reference numeral 16 designates a plurality ofsecondary hot gas injection nozzle (hereinafter referred to simply asnozzles) extending tangentially downwarldly through the wall of theseparating cylinder 10, with one end thereof opening into saidseparating cylinder and the other end into an annular secondary airchamber 12. Reference numeral 13 designates a damper provided in an airduct 17 leading into the secondary air chamber 12, 14 a heater and 15 ablower. Reference numeral 18 designates a ceiling of a separationchamber defined by the separating cylinder 10, 19 a cleaning gas outletport, 20 an exhaust blower, 21 a damper provided in an exhaust duct, 22a seed particle inlet port provided in the peripheral wall of thecylinder 1, 22' a closure lid for the seed particle inlet port, 23 astock solution spray nozzle (hereinafter referred to simply as a spraynozzle), 24 a stock solution tank, 25 a pump and 26 a stock solutionsupply conduit. Reference numeral 27 designates a conically-shapedproduct discharge port (hereinafter referred to simply as a dischargeport), 28 a product discharge conduit (hereinafter referred to simply asa discharge conduit) and 29 a conical valve con centrially disposed inthe discharge port and operated vertically by a valve rod 30 to open andclose said discharge port. Reference numeral 31 designates a hopper, 32a rotary valve and 33 a regulating valve provided in a connecting pipefor regulating the quantity of air supplied into the hopper 31 from ablower (not shown) or the atmosphere. Reference numeral 35 designates astationary guide blade disposed within the tube 9, 36 an auxiliary inletport connected tangentially to the cylinder l for feeding a powder orworking gas I into the cylinder 1 therethrough, the direction of saidjected through the injection nozzles -l6, and 37 a jacket.

The apparatus of the invention constructed as described above operatesin the following manner:

First of all, the blowers 6, 15, the exhaust blower 20, the heaters 5,l4 and means for introducing heat transfer medium into the jacket 37,are actuated to maintain the cylinder at a suitable temperature. Then, aseed powder of the same composition as the final product is fed in asuitable quantity into the cylinder from the inlet port 22. The quantityof the seed powder to be charged is only required to be such that auniform fluidized bed of the seed powder will be formed on theperforated plate 2, usually in a thickness of about 5 30 cm thoughvariable depending upon the size of the cylinder, the physicalproperties of the particle and the design of the perforated plate. Themajor part of the seed powder charged into the cylinderforms thefluidized bed on the perforated plate 2 under the effect of thepressurized air (hereinafter referred to as a primary air) introducedfrom the blowers, and the minor part thereof is suspended in thecylinder. With reference to the primary air, the velocity of the airpassing through the perforated plate 2 is preferably about 2 3 times theminimum air velocity (hereinafter referred to as fluidization startingvelocity) necessary for the fluidization of the particles on theperforated plate 2. Excessively large velocity will result in scatteringof the whole particles upwardly (such velocity will be referred to asend velocity). The fluidization starting velocity and the end velocityare variable depending upon the density, the shape, the dimension, thewater content and the cohesiveness of the particles to be treated. Forsome products, the primary air only is insufficient to form a stablefluidized bed. In such a case, a stirring rod may be provided in thefluidized bed of particles as has been practised in conventionalfluidized drying apparatus. The pressure which is required to beproduced by the blower 6 is the sum of a pressure loss occurring enroute to the perforated plate 2 and the pressure required for theformation of the fluidized bed of particles (hereinafter referred to asfluidized bed pressure loss) which is represented by the followingformula:

AP=PH wherein: AP mm column of water P the apparent specific gravity ofparticle H the height of the stationary bed of particles in terms of mmThus, it will be understood that the power consumption decreases as theheight of the fluidized bed is lowered, but excessively low height tendsto induce a blowthrough phenomenomThe flow rate of the primary air isregulated by the damper 4. The temperature of the primary air is soadjusted by the heater 5 as not to deteriorate the particles and will besuitable for drying. After passing through the fluidized bed, theprimary air ascends in the cylinder L enters the separating cylinder It)through the tube9, moves further upward through said separating cylindetl and is exhausted through b the outlet port 19 to the outside of theexhaust blower 20.

On the other hand, the air blown by the blower l5 (hereinafter referredto as secondary air) is heated by the heater 1 and then introduced intothe secondary air chamber 12 to be injected into the separating cylinder10 through the nozzles 16. Since the nozzles 16 extend through the wallof the separating cylinder 10 obliquely downwardly and tangentially asstated previously, the secondary air forms a swirling downward flow. Asthe quantity of the secondary air is progressively increased, theswirling downward flow moves into the cylinder 1 through the annularpassage 11. As the quantity of the secondary air is further increased,the swirling seocndary air flow moves down to a point adjacent thesurface of the fluidized bed of particles. The downward velocity of theswirling seocndary air flow is reduced by the ascending primary air andfinally becomes zero. Thereafter, the secondary air moves upward,together with the primary air, in the central portion of the cylindertowards the tube 9. In this case, the secondary air still continues itsswirling motion and is vigorously mixed with the primary air. Therefore,all of the particles suspended in the cylinder 1 are subjected to theeffect of the centrifugal force created by the swirling air flow, andthe particles of relatively large mass are urged towards the peripheralwall of the cylinder before they complete their upward movement andagain move downward while being entrained in the swirling downward airflow. The air flow moving upward through the tube 9 is accelerated inits swirling motion by the spiral guide blade 35 and enters theseparating cylinder 10, so that those of the fine particles having arelatively large mass in said separating cylinder are urged towards thewall of said separating cylinder immediately and entrained in theswirling downward flow of the secondary air. The ultra-fine particlescontinue their swirling upward movement but are accelerated by thesecondary swirling air flow as they move upward and urged towards thewall of the separating cylinder, and finally moved back into thecylinder 1 while being entrained in the swirling downward air flow. Thequantity of the secondary air is preferably more than 30 percent of theprimary air, and the velocity of the same when-injected into theseparating-cylinder 10 through the injection nozzles 16 is about 35m/sec or greater. It has been found that when the secondary air isinjected at a velocity of about 60 m/sec, the particle separating effectis greatly enhanced and the particles of 7 microns or larger in size canbe completely separated. Conventional cyclones are capable of completelyseparating particles of 30 40 microns lll size.

In the apparatus of the invention, as described aobve, a fluidized bedof particles is formed at the bottom of the cylinder continuously, whilein the cylinder 1 and the separating cylinder 10, the particles arecontinuously suspended and separated by the effects of the swirlingdownward and upward air flows. Under such condition, the pump 25 isoperated to feed a stock solution to the spray nozzle 23 from the stocksolution tank 241 through the conduit 26, and sprayed in the cylinder 1.The fine particles of the stock solution sprayed are immediately driedby the surrounding hot air but most of them exhibit different.behaviours in the cylinder before they are completely dried. Namely,some solution particles adhere to the ascending seed powder particlesand move upward or downward therewith. Some other solution particlescollide against the swirling downward powder particles and bondtherewith, or, on rare occasions, reach and adhere the wall of thecylinder directly. However, the undried solution particles attached tothe cylinder wall are successively scraped by the swirling downward flowof powder particles and do not accumulate thereon. Still some othersolution particles fall in the undried state into the fluidized bed ofthe powder particles but are dried substantially concurrently by saidfluidized bed which is thick and large in heat capacity, withoutattaching to the perforated plate 2. As will be understood from theforegoing description, the process of drying the solution droplets intosolid particles in the method of this invention is completely differentfrom that in conventional spray drying methods. In the conventionalmethods, the chance of each droplet of a sprayed solution bonding withthe other droplet before it reaches a discharge port of an apparatus isvery slim, and each droplet is dried from the surface thereof whileremaining in its spherical shape by the effect of surface tension, and ashell is formed. The water remaining interior of the shell is evaporatedlast. Therefore, the product powders are hollow and small in bulkdensity, and, therefore, susceptible to breakage. According to theinstant invention, the droplets of the sprayed solution are attached toand dried on the dry powder particle one on another, so that the productparticle is not spherical but porous in shape and large in bulk density.

In the method of this invention, since the undried solution droplets arenot attached directly to the side wall or the bottom of the cylinder andgrown thereon as stated above, the apparatus for practising the methodcan be very small in size as compared with the conventional spray dryingapparatus. According to the conventional methods, for instance, anapparatus having a diameter of about 3000 mm and a height of about 5000mm is required for the treatment of a ceramic solution at the rate of100 kg/hr, whereas according to the instant invention an apparatus of assmall as 800 mm in diameter and 2000 mm in height is sufficient fortreating the same solution at the same rate, i.e. the apparatus cost canbe reduced to about one third of the former.

Further, because of the small size, the apparatus according to theinvention can be easily constructed in double wall structure, and inthis case a highly efiective heat transfer can be achieved between thepowder particles entrained in the swirling air flows and the cylinderwall, by circulating a high temperature heating medium through betweenthe inner and outer walls. Consequently, the thermal efficiency can beenhanced and the drying effect can be improved, and in addition, thecondensation of aqueous vapour on the cylinder wall can be prevented,which would otherwise occur as a result of the aqueous vapour beingcooled from the outside of the cylinder wall, and, therefore, theattachment of the powder particles to the cylinder wall or the growth ofthe same on the cylinder wall can be completely avoided. On someoccasions, it is preferable to circulate a low temperature heatingmedium through the double walls, depending upon the properties of thedesired product. The formation of the fluidized bed of powder particleshas further positive advantages as set forth below, in addition to thoseset forth above. Namely, one of them is that the provision of asecondary dryer becomes unnecessary as the residence time of each powderparticle within the cylinder can be substantially prolonged and thedrying up to a low water content region becomes possible which has beenimpossible heretofore. Another one is that the particle size of theproduct particles can be unifonnalized by the formation of a singlefluidized bed.

As stated previously, the wind velocity which causes the scattering ofparticles in the fluidized bed, that is, the end velocity, is influencedby the density and size of the powder particles. This means that, whenpowder particles of different sizes are fed into the fluidized bed,those of relatively small density and size are readily scatteredupwardly and those of relatively large density and size are fluidized inthe bottom of the cylinder. Therefore, when the product particles arewithdrawn from the bottom of the cylinder, the particles of large sizecan selectively be obtained, i.e. the particles of uniform size andquality can be obtained, and further the flowability of the particle canbe improved.

Still another advantage is that, since the time powder particles areconstantly circulating within the cylinder in a suspended state, towhich the droplets of a sprayed stock solution adhere, a solution suchas a sugar stock solution, from which the product powder can hardly beobtained merely by drying, can be easily converted into powder.Conventional spray drying methods for sugar stock solutions includethose disclosed in Japanese Patent Publications No. 7810/66 and No.20384/69. In

.the method of the instant invention, however, while it may be necessaryto continuously feed a seed powder into the apparatus depending upon theproperties of a stock solution to be processed and the properties of thedesired granule, in most cases the seed powder is required to be fedonly at the start of the operation. Furthermore, the method of theinvention has such advantages over the prior art methods that theapparatus used is simple in construction and the construction cost canbe as low as about one fifth of that of the apparatus used in the priorart methods, that the operation is very easy and that the productioncost can be reduced.

In practising the method of this invention, a screw feeder or the likemay be provided at the bottom of the apparatus for withdrawing theproduct powder, but in this case, means for detecting the level of thefluidized bed must be provided to withdraw the product while maintainingthe height of the fluidized bed at a substantially constant level. In amore effective method of discharging the product powder, the interior ofthe cylinder is maintained at a subatmospheric pressure (in this type ofapparatus, a negative pressure generally appears in the cylinder 1during operation) and the valve 29 and the air regulating valve 33 areopened, whereupon the atmospheric air flows into the cylinder throughthe valve 33, the connecting pipe 34, the discharge conduit 28 and thegap between the wall of the discharge port 27 and said valve 29. Theflow rate of the air introduced into the cylinder is determined by theresistance of the above-described passage and the pressure at thedischarge port. When the seed powder is placed on the perforated plate 2under such condition, the negative pressure at the discharge portbecomes small and the flow rate of the air flowing into the cylinder islowered, due to the increasing quantity of the product particles and thepressure loss of the fluidized particle bed. When the quantity of theproduct particles is small and the flow rate of the air introduced intothe cylinder is high, the product particles are blown up by the air andnot discharged, but when the quantity of the product particles increasesand the flow rate of the air is decreased to a certain level, theproduct particles are suddenly discharged. The product particles thusdischarged are only those whose particle size is large enough to falldown by gravity against the blowup force of air introduced intothecylinder. Those particles which are smaller in size than theaforesaid particles are selectively blown up by the air and notdischarged.

With the arrangement described above, the flow rate of the'air blowinginto the cylinder is automatically increased as the quantity of theparticles on the perforated plate 2- decreases, and hence the productparticles are not discharged. On the other hand, when the quantity ofthe particles increases, the flow rate of the air is automaticallydecreased by reason of pressure balance, and the product particles aredischarged. Thus, it is unnecessary to provide a special levelcontrolling means and the operation is very simple. If necessary, ablower may be provided to blow air into the discharge port 27 at apredetermined rate.

The selection of the particle size of the product particles is possibleby controlling the flow rate of the air introduced into the cylinderasdescribed above. The repeated experiments conducted with a prototypeapparatus have revealed that, if the damper 21 and the air regulator 33are previously adjusted, the level of the fluidized bed cna always bemaintained constant, without fluctuation of the internal pressure of thecylinder and the product particles can be discharged constantly in thenormal state, and thus a very uniform product can be obtained.

A practical example of the method of obtaining a productgranularparticle from a sugar stock solution, using the apparatusdescribed above, will be illustrated hereunder: First of all, kgs of 100mesh castor sugar is charged in the empty drying chamber and then hotair heated at 90C is introduced at the rate of 5 m lmin through airchamber 3 and hot air heated at 100C is introduced as the secondary airat the rate of 8 m lmin through air chamber 12, into the drying chamberby operating the blower which has previously been adjusted. Therefore,the blowers 15 and 6 obviously deliver air at the rate of 13 m /min, butthe drying chamber is maintained at a suitable negative pressure.Lukewarm water at 85C is circulated in the jacket. The castor sugarcharged moves actively in a fluidized state in the bottom of the dryingchamber, with part thereof makes .a circulatory upward and downwardswirling movement. A stock sugar solution, composed of 39.5 percent ofwater and 60.5 percent of solid sugar, is heated to 75C and fed into thedrying chamber at the rate of kg/hr by means of the pump 25. A castolsugar is formed about 10 minutes after the stock sugar solution issprayed. The castol sugar thus formed is discharged from the dryingchamber by opening the valve 29 and the air regulating valve 33 to asuitable degree of opening. Thus, a product granular sugar can beobtained continuously. The rate of discharge of the product particlesduring a short period after the discharge valve is opened, is not equalto the value calculated from the proportion of the solid in the stocksolution supplied, but is naturally stabilized at said value in about 5minutes, and the level of the fluidized bed of castol sugar in thebottom of the drying chamber is also maintained constant. The size ofthe product particles also becomes uniform, once the apparatus hasreached the normal operating condition. The measured particle sizes ofthe product particles discharged in the normal operation of theapapratus are shown in the table below:

Mesh (D) l6l6-20 20-50 50-60 60-80 80 Per 1.8 1.2 87 4 4 2 centage Themethod and apparatus of the instant invention can also very effectivelyused in obtaining a granular powder product by adding a liquid to apowdery substance and acting a reactive gas, such as C1 C0 or $0,, onthe powderysubstance-containing liquid. In such a case, the powderysubstance orthe gaseous reactive substance is continuously fed into thecylinder and caused to makea circulatory upward and downward swirlingmotion of the type described above, while a reactive gas is blow intothe cylinder through the perforated plate, the nozzle or the auxiliaryinlet port to finely atomize liquid in the cylinder. The liquid, thepowdery substance and the reactive gas are sufficiently contacted andchemically reacted with each other, whereby granular powdery particlescan be obtained.

According to the instant invention, as described above, fine particlesare suspended in the drying cylinder by gas blown into said dryingcylinder from the bottom thereof, while liquid of the same material assaid particles is continuously sprayed in the same drying cylinder, thedroplets of said sprayed liquid forming granular particles using saidfine particles as nuclei, said granular particles being retained in thebottom of said drying cylinder in fluidized state under the action ofsaid gas, whereby the particles relatively weakly bonded are partiallydestroyed into fine particles which replenish the consumed fine nuclearparticles in the drying cylinder, and those of said granular particleswhich have grown to a certain desired size being discharged from thedrying cylinder.

The formation of the swirling upward and downward flow of gas in thedrying cylinder is not always necessary where the material to beprocessed is easy to handle and less adhesive, but is essential formaking the apparatus compact and. minimizing the quantity of the fineparticles escaping from the apparatus while being entrained in theexhaust gas. It is also very important that a fluidized bed of particleis formed in the bottom of the drying cylinder. On some occasions,however, the height of the drying cylinder may be increased sufficientlyto ensure that the droplets of the sprayed solution are completely driedbefore they reach the bottom of the cylinder and the fine nuclearparticles are fed in a sufficient quantity into the cylinder through anadditional inlet port. The solution may be sprayed horizontally orobliquely from the wallof the drying cylinder towards the centerthereof, or may be sprayed upwardly from a point adjacent the fluidizedbed at the bottom of the drying cylinder. In short, the solution may besprayed in any manner, provided that the operation of the apparatus willnot be adversely affected or the yield of the product particles will notbe lowered by the droplets of the solution attaching to the peripheralwall or the bottom of the cylinder and growing thereon. It is also to beunderstood that the powderization of a liquid material may be attainedby cooling and solidifyexample illustrated herein.

Although the present invention has been described and illustrated hereinin terms of a specific embodiment thereof, it should be understood thatthe invention is not restricted to the details of the embodiment shownbut many changes and modifications are possible. For-instance, thecylinder 1 is preferably of circular cross-section but may be of apolygonal cross-section. Further, the method of the invention isapplicable, not only to sugar and ceramics, but also to paraffin andother compounds.

We claim:

1. A spray drying method comprising forming a layer of solid particlesat the bottom of a drying chamber defined by a vertical cylinder,blowing gas into said layer from the underside to agitate the whole ofsaid layer and thereby to form a fluidized bed of solid particles withpart of said particles being blown up from the upper surface of saidfluidized bed and suspended within said drying chamber, blowing gas intosaid drying chamber ina direction to form a swirling downward gas flowalong the side wall and a swirling upward flow along the center of saiddrying chamber, whereby said suspending particles are caused to make acirculatory motion while being entrained in said swirling gas flows insuch a manner that the fine particles spirally move upward in the centerof the drying chamber and at least the major part thereof approach theside wall of the drying chamber under the effect of centrifugal forcebefore they reach the top of said drying chamber and finally spirallymove downward along the side wall of said chamber, said particles beingpartially returned to said fluidized bed and partially being carriedupward again in the center of the drying chamber by being entrained insaid swirling upward gas flow, spraying stock solution into said dryingchamber, the droplets of said sprayed stock solution colliding againstand adhered to said circulating particles to form larger sizedparticles, said larger sized particles being dried by the surroundinggas and partially circulated within said drying chamber and partiallyfalling into said fluidized bed, and discharging said larger sizedparticles from said fluidized bed while maintaining the depth of saidfluidized bed substantially constant.

2. A spray drying method according to claim 1, wherein said gas isintroduced into said drying chamber from product particle discharge portmeans provided at the bottom of said drying chamber and those of saidlarger sized particles which have grown to a predetermined size andlarger are discharged under gravity through said discharge port meansagainst the flow of said gas, whereby the particles of the predeterminedsize are selectively obtained.

3. A spray drying method according to claim 1, wherein said dryingchamber has a double wall structure and a heat transfer medium iscirculated through the interspace between the inner wall and outer wall.

4. A method in accordance with claim 1 in which the gas is introducedinto the drying chamber in the direction substantially tangential to theside wall of said chamber.

5. A method in accordance with claim 1 in which said swirling upwardflow along the center of said drying chamber is passed throughseparating means to centrifugally separate the particles suspendedtherein.

6. A spray drying apparatus comprising a vertical cylinder defining adrying chamber, gas inlet port means provided at the upper portion ofsaid drying chamber to form swirling downward gas flow, gas outlet portmeans provided at the center of the upper end of said drying chamber forexhausting an'ascending gas flow therethrough, perforated plate meansdisposed at the bottom of said drying chamber, means for blowing airinto said drying chamber from the underside through said perforatedplate, means for spraying stock solution into said drying chamber, andmeans for discharging solid particles from a solid particle layer formedon said perforated plate means.

7. A spray drying apparatus according to claim 6, wherein said dischargemeans includes at least one product discharge conduit extending upwardlytowards the bottom of said drying chamber with the upper end opening atsaid perforated plate, and is provided with means for regulating gasflow being introduced into the drying chamber through said dischargeconduit.

8. A spray drying apparatus according to claim 6, wherein means isprovided at the top portion of said drying chamber for separating thesolid particles by the effect of centrifugal force from the gas flowingupwardly from said drying chamber and part of swirling gas flow withinsaid separating means is led into the drying chamber to form swirlingdownward gas flow.

1. A spray drying method comprising forming a layer of solid particlesat the bottom of a drying chamber defined by a vertical cylinder,blowing gas into said layer from the underside to agitate the whole ofsaid layer and thereby to form a fluidized bed of solid particles withpart of said particles being blown up from the upper surface of saidfluidized bed and suspended within said drying chamber, blowing gas intosaid drying chamber in a direction to form a swirling downward gas flowalong the side wall and a swirling upward flow along the center of saiddrying chamber, whereby said suspending particles are caused to make acirculatory motion while being entrained in said swirling gas flows insuch a manner that the fine particles spirally move upward in the centerof the drying chamber and at least the major part thereof approach theside wall of the drying chamber under the effect of centrifugal forcebefore they reach the top of said drying chamber and finally spirallymove downward along the side wall of said chamber, said particles beingpartially returned to said fluidized bed and partially being carriedupward again in the center of the drying chamber by being entrained insaid swirling upward gas flow, spraying stock solution into said dryingchamber, the droplets of said sprayed stock solution colliding againstand adhered to said circulating particles to form larger sizedparticles, said larger sized particles being dried by the surroundinggas and partially circulated within said drying chamber and partiallyfalling into said fluidized bed, and discharging said larger sizedparticles from said fluidized bed while maintaining the depth of saidfluidized bed substantially constant.
 2. A spray drying method accordingto claim 1, wherein said gas is introduced into said drying chamber fromproduct particle discharge port means provided at the bottom of saiddrying chamber and those of said larger sized particles which have grownto a predetermined size and larger are discharged under gravity throughsaid discharge port means against the flow of said gas, whereby theparticles of the predetermined size are selectively obtained.
 3. A spraydrying method according to claim 1, wherein said drying chamber has adouble wall structure and a heat transfer medium is circulated throughthe interspace between the inner wall and outer wall.
 4. A method inaccordance with claim 1 in which the gas is introduced into the dryingchamber in the direction substantially tangential to the side wall ofsaid chamber.
 5. A method in accordance with claim 1 in which saidswirling upward flow along the center of said drying chamber is passedthrough separating means to centrifugally separate the particlessuspended therein.
 6. A spray drying apparatus comprising a verticalcylinder defining a drying chamber, gas inlet port means provided at theupper portion of said drying chamber to form swirling downward gas flow,gas outlet port means provided at the center of the upper end of saiddrying chamber for exhausting an ascending gas flow therethrough,perforated plate means disposed at the bottom of said drying chamber,means for blowing air into said drying chamber from the undersidethrough said perforated plate, means for spraying stock solution intosaid drying chamber, and means for discharging solid particles from asolid particle layer formed on said perforated plate means.
 7. A spraydrying apparatus according to claim 6, wherein said discharge meansincludes at least one product discharge conduit extending upwardlytowards the bottom of said drying chamber with the upper end opening atsaid perforated plate, and is provided with means for regulating gasflow being introduced into the drying chamber through said dischargeconduit.
 8. A spray drying apparatus according to claim 6, wherein meansis provided at the top portion of said drying chamber for separating thesolid particles by the effect of centrifugal force from the gas flowingupwardly from said drying chamber and part of swirling gas flow withinsaid separating means is led into the drying chamber to form swirlingdownward gas flow.